Automatic glide and/or climb control



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April s, 1943. c. J. CRANE E'T AL 2,315,501

AUTOMATIC GLIDE AND/OR CLIMB CONTROL Filed Jan. e, 1941 s sheets-sheet 2 62 f so 7a Pan/fe l www 1 l 3 5N 5 1 3 y m, @a M M im M 2, u m co5 w di. 5. Z 6 in@ im 6? C J CRANE ET AL AUTOMATIC GLIDE AND/OR CLIMB CONTROL Flled Jan 6, 1941 70 Powie .50p/q1 April 6, 1943.

Patented Apr. 6, 1943 AUTOMATIC GLIDE AND/ OR CLllVIB CONTROL Carl J. Crane, Shreveport, La., and George V. Holloman and Raymond K. Stout, Dayton,

Ohio

Application January 6, 19AM, Serial No. 373,334

(Granted under the act `of March 3, 1883, as

amended April 30, 1928; 370 O. G. 757) 4 Claims.

The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to us of any royalty thereon.

This invention' relates to mechanism for controlling the rate of ascent or descent of an aircraft, so that the said rate of ascent or descent remains substantially constant at a preselected value. The invention more particularly relates to a sensitive control means associated with the well-known automatic pilot, such that the longitudinal control device of the automatic pilot is adjusted by a rate-oichangeofaltitude responsive means to maintain level flight, or to ascend, or descend, at a constant preselected rate. The device in accordance with the invention is particularly adapted to control the rate lof descent of an aircraft during the course of an instrument landing, made in accordance with the well-known Army Air Corps blind landing system, or modifications thereof, in which the aircraft is directionally aligned with the axis of the landing rune way by signals received from respective radio compass transmitting stations spaced along the axis of the runway; and in which the descent is made in one or more stages at a constant rate from a predetermined altitude until contact is made with the ground. In order to releive the pilot of the necessity of manually controlling the attitude of the aircraft as well as the engine throttle, in order to descend in a power glide at a constant rate, the throttle is set for an engine speed giving the correct power output for the glide and the auto` matic pilot maintains the aircraft in the level flight condition and by means of the sensitive i control device` in accordance with the invention, the automatic pilot is caused to actuate the' elevators of the vaircraf t to correct for any departure in the rate of descent from the preselected value.

In a known prior art device disclosed in United States Patent No. 2,091,306, granted to Bert G. Carlson, it has been proposed to control the asf. cent, descent, or level flight conditions of an aircraft, by an automatic pilot having associated therewith a rate-of-change-of-altitude reponsive means. In this device a rate of climb or descent responsive device, is operative to control a valve, which in turn controls the supply of air directed by nozzles on to each of a pair of oppositely rotating turbine wheels, each turbine being connected to operate a common shaft in opposite directions respectively. The shaft in turn is operative to introduce a control correction factor into the automatic pilot. As long as the torqucs on the turbines remain equal, no correction is introduced Cil ' into the automatic pilot, but upon a departure of the aircraft from a preselected rate of ascent or descent, the rate-of-change-of-altitude responsive means causes the control` valve to alter the supply of air to the respective turbine nozzles, to thereby cause a greater torque on one turbine than on the other, thus causing a rotation of the adjustment shaftin the direction of the greater torque acting thereon. 4The rotation of the adjustment shaft acts on the automatic pilot to cause the elevators to move up or down to restore the aircraft to the desired condition of flight.

The patented device in actual practice was however found to be entirely unsuited to accomplish the primary function of the present invention i. e. to accurately maintain the rate of climb or descent of an aircraft constant, at values of climb or descent respectivelyiemployed in takeoff, or in instrument landing of the aircraft. The lack of sensitivity in the functioning of the patented device for the control of the aircraft rate of climb or descent, was found to be primarily due to two separate causes. The first cause being, that the pressure difference existing in a rate-ofclimb meter available to operate the indicating elements thereof, never exceeds a value of approximately one inch of water and any friction or other load imposed on the pointer or indicating mechanism causes large errors in the rate of response end magnitude of indication of the instrument. To attempt to actuate a valve or other mechanical control element from the indicating mechanism of a rate-of-climb indicator destroys the possibility of obtaining a sensitive control due to the inertia and friction loads imposed on the indicating mechanism. The second defect was determined as being due to the fact, that a considerable excess of torque had to be developed by one of the turbine wheels in order to overcome the effects of inertia and friction in the adjustment mechanism, with the result that the adjustment when once initiated proceeded at a very high rate Aof-speed causing overshooting of the control with consequent hunting.

i In order to overcome the defects of the known prior-art devices and provide a rate-of-changeof-altitude responsive control means operative to automatically maintain the rate of climb or descent ,of an aircraft substantially constant, or to maintain level flight at a preselected altitude the present invention provides an electrical servo means for introducing the'control correction into rthe automatic pilot, the said electrical servo means being controlled by a photoelectric means. The photoelectric means is rendered operative by a beam, or beams of light, controlled by a rateof-'change-of-altitude responsive device, in accordance with the variation in the said rate-ofchange-of-altitude from a predetermined value. By employing a photoeleetric control means, no load is imposed on the rate-of-change-of-altitude responsive means and the responsive means is thus enabled to accurately respond to therateof-change-of-barometric pressure resulting from the ascent or descent of the aircraft and to exert a control in accordance therewith. To prevent hunting, the control device in accordance with the invention is provided with damping means, which prevents a too rapid application' of the control and provides for a gradual dimunition of the control effeet, so that the control effect is substantially zero, when the aircraft is returned to the desired rate of ascent or descent.

The principal object of the invention is the provision in combination with an aircraft longitudinal control system, of power means for actuating said control system, of a control element operative tocontrol the power means to vary the rate of ascent or descent of the associated aircraft, of photoelectrically controlled means for actuating said control element; and rate-of-change-of-altitude responsive means operative upon the variation in the rate-of-ehangeof-altitude from a preselected value for controlling the photoelectric means.

A further object of the invention is the provision in an automatic control system of the character described, of damping means cooperating with the control means to eliminate overcontrolling and hunting.

Other objects of the invention not specifically enumerated will become apparent by reference to the specification and the appended drawings in which:

Figure 1 is an assembly View partly in section, illustrating the invention as applied to a conventional automatic pilot for controlling the ascent or descent of an aircraft;

Figure 2 is a view partly in section, illustrating the novel photoelectric control device in accordance with the invention;

Figure 3 is a front elevation of the rateof change-of-altitude responsive means illustrated in Figure 2;

Figure 4 illustrates a modified form of the photoelectrie control device illustrated in Figure 3; and

Figure 5 is a front elevation of the rate-ofehange-of-altitude responsive means illustrated in Figure 4.

Referring now to Figure l, the reference numeral I generally indicates the longitudinal control unit of a conventional gyroscopically controlled automatic pilot, having a easing 2 adapted to be continuously evacuated by means of a pump, not shown. A conventional air driven gyro rotor indicated by the reference numeral 3 is mounted within the casing 2 and freely rotatable about the shaft 4 which is transversely journaled in a gimbal ring 5 which in turn is freely rotatable about a longitudinal axis. The rotor of the gyro is adapted to be driven by means of air jets in a well known manner, and when rotating at high speed the gyro rotor tends to remain horizontal irrespective of the instant attitude of the aircraft and the casina' 2. The shaft 4 has mounted thereon a valve member 6 which is rotatable therewith and adapted to cooperate with a member 'I rotatable with the casing 2 and having valve ports 8 and 9 which cooperate with the valve edges of the valve member 6. Any pitching of the aircraft about a transverse axis will cause the casing 2 and member 'I to rotate relative to the valve member 6 which is maintained stationary by the gyroscope, and will thus cause an increase in the opening of one and a decrease in the opening of the other-of the ports 8 and 9 which normally are partly and equally in communication with the reduced pressure within the casing 2. 'Ihe ports 8 and 9 areI connected by means of flexible conduits I0 and II to a double acting diaphragm type servomotor generally indicated by the reference numberal I2 having a diaphragm I3 thereof movable in opposite directions upon the increase or decrease of pressure in one of the conduits I0 and II. The chambers formed on opposite sides of the dlaphragm I3 are respectively connected to the conduits I0 and II and are also connected to the outside atmosphere through small restricted bleed ports. The action is such that if the port 8 is increased in open communication with the reduced pressure within-the easing 2, a greater suction will be produced in the conduit I0 than in the conduit II, so that the pressure above the diaphragm I3 will cause a downward movement of the diaphragm and of the valve rod I4 connected thereto. In a similar manner if the valve member 6 moves relative to the valve member 'I so that the opening of the port 9 is increased, the diaphragm I3 will move in an upward direction. The control rod I4 is adapted to actuate a pilot valve not shown, axially slidable within the valve housing I5 and adapted upon movement in either direction from a neutral position to 4admit fluid pressure from a pump I6 to a servomotor II which is connected by means of cables I8 to actuate the aircraft elevators 2U. Any moyement of the piston of the servomotor I7 is transmitted through the follow-up cables 2| to a rotatable drum 22 which is adapted to cause a follow-up movement of the valve member 'I with respect to the valve plate 6, as hereinafter more fully described. The valve member 1 is provided with an extension arm 23 which is pivotally connected by means of links 24 and 25 to be actuated by a gear 26 to cause rotary movement of the valve member 'l relative to the valve plate 6. 'I'he gear 30 meshes with a gear 21 whiehis adapted to be rotated by means of a bevel gear sector 28 and a bevel gear 29 from a shaft 30. The shaft 30 is connected to the planet wheel carrier of a conventional bevel gear differential generally indicated by the reference numeral 32. The shaft 30 through the differential 32 is adapted to be rotated by means of a shaft 33 connected to thel follow-up cable drum 22, so that the valve member 'I is moved back towards its normal neutral position with respect to the valve member G as the servomotor II lactuates the aircraft elevators` This follow-up action is necessary in order that the movement of the elevators be proportional to the angular deviation of the aircraft about a transverse axis so that when the aircraft is restored to the normal level flight position through'the action of the elevators, the elevators will be in the neutral position. The shaft 30 which controls the movement of the valve member 'I as. above explained may also be shifted in either direction independent of the follow-up shaft 33 by means of a gear 3l connected to the third leg of the differential 32, and rotatable by means of a worm not shown,

which is actuated by the bevel gears from a shaft 36. The shaft 36 is adapted to be manually rotated bymeans of a setting knob 31 so that the normal neutral position of the ports 8 and 9 of the valve member 1 relative to the valve member 6 may be shifted in either direction so as to cause the aircraft to ascend or descend at some given angle with respect to the gyro base line determined by the gyro unit 3. The usual arrangement of the gearing con# necting the setting knob 31 and shaft 36 lto the differential 32 is such that clockwise rotation of the setting knob causes theaircraft to descend while a counterclockwise rotation of the setting knob 31 causes the aircraft to ascend.`

With the setting knob 31 set in the level flight position. the gyro 3 through the valve members 6 and 1 is adapted tomaintain the aircraft in the level flight position by correcting any deviation of the aircraft in a vertical plane from the level flight position by a contrary movement of the elevators 2l), which thus restore the aircraft to its normal level flight position, and by rotation of the setting knob 31 in either direction. the aircraft may be caused to ascend or descend at ar fixed angle of inclination with respect to the gyro base line, the gyro however being adapted to maintain the flight attitude of the aircraft along the axis of ascent or descent. for controlling elevators of an aircraft is per se well known in the art and its specific construction forms no part of the present invention.

In an automatic pilot of the character above described there isno provision for controlling the rate of ascent or descent of the aircraft so long as the aircraft remains in the level flight position or ascends or descends along an axis determined by the setting knob 31, and the provision of 'a sensitive control of this character forms the/novel feature of the invention, and is carried out by providing a bevel gear 38 on the manual setting shaft 36 which meshes with a corresponding bevel gear 39 secured on the lower end of a shaft 40 which is rotatably journaled in the casing 2. An automatic means for rotating the shaft 48 in the proper direction forms the means for maintaining the rate of ascent either substantially zero or at some preselected value.

The shaft 40 has mounted thereon and rigidly secured thereto, a clutch element 42 which isadapted to be engaged by an axially slidable clutch element 43 which is slidably kcyedto the shaft 40 and axially movable relative thereto. The clutch member 43 is rigidly connected to a gear 44 also splined or keyed to the shaft 40 but axially slidable thereon. The clutch 43 is adapt ed to be engaged or disengaged with the clutch member 42 by means of a shifter fork 45 actuated by a manual control rod 46, the purpose of the clutch being to connect or 4disconnect the automatic control at will. The gear 44 is in continuous meshing engagement with the teeth of a rack 48 formed as a part of the horizontal ccntrol rod 49 which is adapted to be shifted in opposite directions respectively by means of solenoids 50 and 68 which in turn are adapted to be energized through the medium of a power control relay 10 in turn operatively controlledby a rateof-change-of-altitude responsive device generally indicated by the reference numeral 15. Upon engagement of the clutch elements 42 and 43 axial movement of the control rod 49 in either'direction, will cause rotation of the shaft 40 and set- The above described automatic means automatic pilot to thereby cause the elevators 20 of the aircraft to be displaced in a direction such as to restore the rate of climb or descent of the aircraft to a preselected value.

Referring now to Figure 2, it is seen that the solenoid 5U comprises a winding 5l arranged on an annular core member 52 which has its central bore closed at one end to form a cylinder communicating with the atmosphere through a restricted adjustable bleed port 53. A solenoid plunger member 54 made of magnetic material and `formed as an enlarged head on one end of the control rod 49 extends within the cylinder formed by the core member 52 of the solenoid 50 and engages a compression spring 55 which yieldingly opposes inward movement of the plunger member 54 when the winding 5l is energized. The solenoid is of similar construction to that of the solenoid 50 and comprises va plunger 55 formed on the other` end of the control rod 59 and axially slidable within the cylinder formed by the annular core member 56, the cylinder being in restricted communication with the atmosphere through the adjustable bleed port 51, and inward movement of the plunger 55 being yieldingly opposed by compression spring 58. The solenoid winding 59 is arranged on the core member 56 in concentric relation with the plunger 55.

One of the terminals of each of the solenoid windings 5I and 59 is adapted to be connected by means of a conductor 62 to a suitable power supply not shown. The other terminal of the ,solenoid winding 5l isconnected by means of a conductor 63 to a stationary contact 65 of a power control relay generally indicated by the reference numeral 1U, and the other terminal of the solenoid winding 59 is similarly connected by means of the conductor 64 to a stationary contact 66 of the power supply relay 10. A double contact 61 mounted on a pivoted relay arm 68 is adapted to engage either of contacts 65 or 56 to connect the same to a power supply conductor 69. The

' relay arm'68 is normally maintained by means ting shaft 36, to introduce a correction into the of springs in a neutral position such that the double contact 61 is out of engagement with either of the contacts 65 or 56, but is adapted upon either of the relay coils 1l or 12 being energized to be deflected from a neutral position to cause engagement between contact 61 and either of contacts 65 or 66 respectively. The relay coil 1I is adapted to be energized through conductors 13 by a photoelectric cell 11 of the Photronic type forming .a part ofI a rate-of-change-ofbarometric pressure responsive device 15, commonly known as a rate-of-climb indicator. The relay coil 12 is similarly connected by means of. conductors 14 to a photoelectric cell 16 of the Photronic type also associated with the responsive means 15. The' rate-ofchange-ofbarometric pressure responsive device 15. comprises the casing having a partition wall 18 which incloses a mechanism of a well known sensitive rate-ofcumb indicator, having the indicator shaft 19 operative to move a pointer 88 relative to a dial 82, the pointer 88 having an arcuate sector or shield 8l secured thereto and rotatable therewith (see Figure 3). The dial 82 is provided with an arcuate transparent section 83 through which light from -a respective pair of light sources 84 and 85 may pass therethrough. The light sources 84 and 85 are conventional electric lamps suitably supported in front of the instrument cover glass on a bracket 86 which has one leg. thereon extending through an arcuate slot 81 in the instrument casing and secured to a sleeve 88 rotatably mounted on an enlarged head 89. The bracket 86 also serves as a supporting means for the photo sensitive cells 16 and Il so that the cells are maintained in a position behind the dial 82 directly opposite the respective light sources 84 and 85. The bracket 86 is adapted to be rotatably positioned so that the light sources 84 and 85 are spaced at equal distances on opposite sides of a line intersecting the circumference of the dial 82 at a desired rate of climb or rate of descent indication, within certain predetermined limits. It will be seen by the inspection of Figure 3 that if the desired rate of descent be set, for example, at a value of 400 feet per minute that the shield 8l carried by the pointer 8U will intercept ene-'half of the quantity of light transmitted by the lamps S4 and 85 through the transparent sector 83 of the dial 82 to the respective photo cells T6 and 1l causingeach of the cells to generate an equal current in the respective relay coils 'H and 'i2 so that the power relay armature 68 remains in its neutral position. If, however the aircraft should be descending at a rate greater than the preselected value of 400 feet per minute as seen in Figure 3, a greater quantity of light will be transmitted from the light source 85 to the photoelectric cell 11, than will be transmitted tc the photoelectric cell 'I6 from the light source 84, causing the relay coil Il to attract the relair arm 68 to cause engagement'between the confacts 6T"and 65 and thereby energizing the solenoid 50. The solenoid 50 will pull the plunger 54 axially to the right at a rate determined by the compression spring 55 and bleed port 53. In-

ward movement of the plunger54 will cause the control rod 59 and rack 48 to rotate the gear 44 in a clockwise direction, as seen in Figure 2, which will cause rotation of the shaft 40 in a like sense and cause a corresponding counterclockwise rotation of the manual setting shaft 36 of the automatic pilot (Figure 1). This rotation of the setting shaft 36 will cause the elevators 20 of the aircraft to move up tending to reduce the rate of descent until the preselected value of 400 feet per minute is again obtained. If` the aircraft should tendl to descend at a rate less than the desired 400 feet per minute (Figure 3), the photo cell 16 will receive an increased amount of light from the light source 84 sufficient to cause the relay coil 'l2 to move the relay arm 68 in the opposite direction to cause engagement of contacts 66 and 61 thereby energizing the solenoid 60. Upon the solenoid 60 becoming energized, the control rod 49 will cause axial rotation of the `shaft 40 and the manual setting shaft 36 of the automatic pilot (Figure 1) in the opposite direction from that above described, causing the elevators 20 of the aircraft to be deected downward to increase the rate of descent until the preselected value is again reached.

It will be seen from the description of the operation of the novel photoelectric control system that any desired rate of ascent or descent including level flight may be maintained by setting the bracket 86 in the desired position, and that thereafter the rate-of-climb indicator will actuate the manual setting shaft 36 of the automatic pilot to maintain the desired flight condition. The provision of the dash pot cylinders in the so-lenoids 50 and 60 (Figure 2) tends to prevent a too rapid introduction of the correction factor into the automatic pilot due to the resistance offered to the discharge of air from the solenoid cylinders by the respective bleed ports 53 and :T and permitting the correction to be introduced at a rate such. that hunting will be avoided. The dash pots also cause the control effect to be canceled gradually rather than abruptly which further eliminates undesirable hunting effects.

In employing the structure disclosed in Figures 1 to 3 inclusive, for example, to maintain level night, the pilot places the automatic control in operation by actuating the clutch control rod 46 to cause engagement of the clutch elements 42 and 43 and then sets the adjustable bracket 86 on the rate-of-climb indicator so that with the pointer in the zero position the shield 8| attached thereto will intercept equal quantities of light from the light sources 84 and 85. If then the aircraft should either ascend or descend even though the aircraft remains in the horizontal position, will cause the rate-of-climb indicator to indicate ascent or descent and apply a correction to the automatic pilot to overcome the tendency to ascend or descend. Since no load is imposed on the operating mechanism of the rate-of-climb indicator, a very sensitive response may be obtained, thus overcoming the difficulties encountered with prior art control structures of a similar nature. When it is desired to descend, for example, during the course of an instrument landing at a predetermined rate of descent, the pilot closes the engine throttle so that the aircraft enters a power glide with a rate of descent of approximately the desired amount. While still maintainingA a level flight attitude, the pilot having previously set the adjustable bracket 86 of the rate-of-change-of-altitude device at the value corresponding to the desired rate of descent, for example, 400 feet per minute, the sensitive control means will then be operative to maintain this rate of descent substantially constant by minor adjustment ofthe aircraft elevators in either direction through the medium of the automatic pilot. It is thus not necessary for the pilot having once set the'engine throttle in a predetermined position to further exert longitudinal control onthe aircraft until a landing has been accomplished. The device of Figures 1 to 3 is similarly adapted to control the aircraft during take-off to maintain the rate of climb substantially constant at a preselected value.

Figures 4 and 5 illustrate a modied form of photoelectric control which differs from the device illustrated in Figures 1 to 3 in that the photo electric cells are each operative to control the aircraft in a respective flight condition but are not jointly operative. In this modification, the photo cells 16 and l1 are mounted in a fixed relation behind the dial 82 of the rate-of-changeof-altitude responsive device 15', and the light sources 84 and 85 are similarly fixed in front of the instrument dial. The photoelectric cells 16 and 'l1 are adapted to receive light from the respective light sources 84 and 85 through suitable apertures cut; in the dial 82, the light being intercepted by means of an arcuate shield 8| of the same character as the shield in the device illustrated in Figures 2 and 3. The power relay generally indicated by the reference numeral 90 has its contacts 9| and 92 respectively connected to the conductors 63 and 64 of the respective solenoids 58 and 60. The relay 98 is provided with a double contact 93 carried by a pivot'ed relay arm 94 and adapted to engage either of stationary contacts 9| or 92 respectively. The relay arm 94 is normally biased by a light tension spring 95 so that the contacts 92 and 93 are normally engaged. The relay 90 is also provided with a relay coil 96 which is adapted to be connected by means of conductors 91 parallel to respective pairs of conductors 98 and 99, the conductors 98 being connected to the photo cell 86 and conductors 99 being connected to the photo cell 11. Switches |00 and IUI permit either of the photo cells 16 or 11 to be electrically connected to the relay coil 96. Relay arm 94 of the relay 90 is connected-to the power supply conductor 69,. as in the device of Figures 1 to 3 inclusive. c

In order to control the aircraft in the normal level flight condition to prevent an ascent or descent thereof, the photo cell 11 and light source 85 are placed in a position such that with the rate-of-climb indicator pointer 90 indicating zero rate of climb or descent, the edge of the shield 8l will intercept approximately one-half of the light transmitted from the light source 95 to the photo cell 11. If then the switch IUI be closed, the current passing from the photo cell 11 to the relay coil 96 will be suiilcient to pull the relay arm 94 to a neutral position yagainst the resistance of the spring 95 so that the contact 93 is out of engagement with either of the contacts 9i or 92. 1f then the aircraft should tend to descend, the shield 8| will move in a position admitting more light from the source 85 to the photoelectric cell 11, and causing a current to flow' in the relay coil 96 sufficient to attract the relay arm 94 and cause engagement between the contacts 9i and 93. Engagement of the contacts 9i and 93 will energize the solenoid 50 causing the control rod 49 to be moved axially to the right and introdueing a correction into the automatic pilot through the gear 44 inthe same manner as in the device of Figure 1 yto cause the aircraft elevators to be moved upward to compensate for the descent until the aircraftis again the normal level flight condition. If the aircraft should tend to ascend, the shield 8l attached to the pointer 80 will move to cut oir the light passing from'the light source 85 to the photoelectric cell 11 and reducing the current now through the relay coil 96 an amount sufficient to allow the spring 95 to pull the relay arm 94 to the right causing engagement between the contacts 92 and 93 to thereby energize the solenoid 60 and to introduce a compensating correction into the automatic pilot in the same manner as in the device of Figures 1 to 3 inclusive.

In order to, for example, control the rate of descent of the aircraft during the course of an instrument landing, the light source 84 and photo cell 16 are so positioned, that the edge of the shield 8| intercepts approximately one-half of the light transmitted from the light source to the cell when the pointer 80 indicates the desired rate of descent, for example, 400 feet per minute (see Figure 5). The rate-of-climb indicator 15' is then operative to control the relay 90 to maintain a predetermined rate of descent in the same manner as above described with reference to the level flight condition. When it is desired to control the aircraft during descent, the switch IM is opened and the switch |00 is closed so that the photoelectric cell 18 is operative to energize the relay coil 99 of the sensitive relay 90. In a similar fashion additional light sources and photo cells may be positioned in order to control other .conditions of night, for example, rate of climb scope of the invention as defined by the appended claims.

We claim: 1. In anautomatic pilot having a primary control means for normally maintaining an aircraft inlevel flight and a modifier for said primary control yfor causing the aircraft to ascend or descend; of means for automatically actuating said modifier in accordance with the variation in the rate of change of altitude from a predetermined value comprising, a reversible power means forl actuating said modifier, a relay for selectively controlling the energizing of said reversible power means, and avcontrol means for said relay including a source of light, a photoelectric control device adapted to respond to light received from said source and means responsive4 to rate of change of altitude for controlling the quantity of light transmitted from said source to said photoelectric control device upon a departure of the rate of change of altitude from a predetermined value.

2. The structure as claimed in claim l, in which the means responsive to rate of change of altitude includes a pressure responsive means responsive to the rate of change of barometric pressure as the same changes with altitude, a rotatable shieldI interposed in the path of transmission of light from said light source to said photoelectric device and operative upon rotation thereof in either direction from a predetermined position to vary the quantity of said transmitted light, said shield being operatively connected to said pressure responsive means and rotated thereby from said predetermined position upon a variation in the rate of change of altitude from said predetermined value.

3. The structure as claimed in claim l, in which the photoelectric deviceincludes a -pair of photoelectric cells adapted to receive light from said light source, and a rotatable shield interposed in the path of light from said light source to said cells and operable upon rotation thereof in either direction -to differentiallyv vary the amount of light transmitted to the respective photoelectric cells, said shield being operatively connected to the said means responsive to rate of change of altitude and rotatable thereby in accordance with the variation in said rate of change of altitude from a predetermined value.

4. The structure as claimed in claim 1, includ- 'ing adjustable means associated with said rate of change of altitude responsive means for preselecting the rate-of-change-of-altitude above and eelow which said responsive means becomes operable to cause selective actuation of said photoelectric device.

CARL J. CRANE. GEORGE V. HOLLOMAN. RAYMOND K. STOUT. 

